1. Field of the Invention
The invention relates to a tunnel kiln for firing ceramic porous bodies containing organic binders, and more specifically to a carriage type tunnel kiln suitable for firing a ceramic honeycomb structures.
2. Description of Related Art
In a tunnel furnace for firing ceramic porous bodies containing organic binders such as ceramic honeycomb structures, its preheating zone is subject to generation of a large amount of organic binder gases. Conventionally, the organic binder gases are burned at a high air ratio by using high-speed burners capable of giving high-velocity flame to supply the preheating zone with a large amount of combustion gas in order to enhance the effect of stirring so that the temperature distribution may be kept well. Besides, a circulation fan has been fitted to the preheating zone to improve the effect of stirring. The technology for homogenizing the in-furnace temperature distribution is available, however still there is a problem that productivity cannot easily be improved because binder releasing process in the preheating zone takes a long time. The reasons are as follows.
That is, since an organic binder gases generating in the preheating zone burns on the surface of a ceramic porous body, its periphery is heated to a high temperature, whereas its inside stays at a comparatively low temperature because of its excellent thermal insulation properties. Subsequently, as the temperature of the inside of the ceramic porous body rises, combustion of the organic binder gases go on to heat the inside to a high temperature, whereas the temperature of the periphery does not readily rise because of the excellent thermal insulation properties of the ceramic porous body. For this reason, a difference in temperature may occur between the inside and the periphery during a binder releasing process, leading to the occurrence of cracks referred to as breaks due to thermal stress.
Conventionally, to avoid the problem, the progress of the binder releasing process is moderated by elongating the preheating zone and slowing down the traveling speed of a carriage.
Patent Documents 1 and 2 disclose a tunnel furnace that suppresses combustion of organic binders by supplying nitrogen gas etc. to a preheating zone to decrease an oxygen concentration in an atmosphere and thereby complete releasing of binder in a short period of time. However, this requires a large amount of nitrogen gas, leading to a problem of higher running costs. There is also another problem that the tunnel furnace is subject to entry of fresh air when a carriage is being carried in. Consequently, is difficult to keep the oxygen concentration in the preheating zone at a level of 8% or less, or preferably 5% or less at which the breaks never occur as described above. For those reasons actually the temperature is allowed to rise gradually over a long period of time in order to prevent breaks from occurring in the binder releasing process.